1. Field of Invention
This invention pertains to computerized graphics, and particularly to a computerized graphics tool that facilitates depiction of movement of a displayed object.
2. Related Art and Other Considerations
Early computers dealt primarily with the processing and display of text and numbers, such as word processing and mathematical computation. As computers became more sophisticated, and as the pictorial quality of display screens (such as the computer monitors) improved, the capability of computers increased, and an entire computer graphics industry emerged.
Computer graphics enables simulation, on a display screen, of a displayable object. For example, a computer graphics system enables a person such as a designer to generate an image of a 3-D object, such as a chair, for example. Most computer graphics systems allow the designer then to move the image of the chair on the screen, even permitting the designer to turn the chair around, to turn the chair upside down, or to rotate the chair about any of several possible axes.
The ability to move a displayed object is advantageous in computerized graphics. Such movement enables the viewer to see differing aspects of the displayed object. Being able to see graphically the displayed object from other perspectives is important, for example, when a particular surface or portion of the displayed object must undergo further graphical treatment or embellishment. It is also important in a situation in which the displayed object is to form just one building blocking in an overall collection of displayed objects. The ability to visualize differing surfaces of such an object helps a designer better understand how the object is to be connected or situated relative to other objects, as occurs in CAD CAM designs, for example.
For sake of showing rotation of a displayed object, some computer graphics systems depict the displayed object on a screen with the displayed object being pictured within a circle. Such circle-based systems typically have a user input device, such as a mouse or thumbwheels, which controls the position of a pointer or some other referencing frame on the screen. In at least some modes of operation, movement of the user input device, and consequentially of the pointer or frame, causes a rotation or a change of vantage point of the displayed object.
Two examples of computer graphics systems which depict a displayed object within a circle are shown in U.S. Pat. No. 4,734,690 to Waller and U.S. Pat. No. 5,019,809 to Chen. The Chen system is also described to some degree in Chen et al., "A Study In Interactive 3-D Rotation Using 2-D Control Devices", Computer Graphics, Vol. 22, No. 4, August 1988, pp. 121-129. These example systems purport to provide either a spherical panning or visual spherical encapsulation of the displayed object. Waller provides a viewing window which is positionable relative to an imaginary sphere. An imaginary sphere is described by a circle; the window is in a plane tangent to the sphere depicted by the circle. Chen provides a reference circle within which the displayed object is encapsulated, supposedly for acquiring a "trackball" feel.
Some circle-based graphics schemes have not proven sufficiently user friendly. The system of Chen particularly has been criticized, among other things, for not providing sufficient visual feedback and for having a behavior that is hard to understand. See Shoemake, Ken, "Arcball: A User Interface for Specifying Three-Dimensional Orientation Using a Mouse", Proceedings Graphic Interface 1992; May 11-15, 1992; pp. 151-156. Since Chen's reference circle is strictly two-dimensional, its appearance does not change when the displayed object is rotated. While this has the advantage of allowing the circle to be painted on a table or the like instead of being displayed, it does not give the user the feel of a three-dimensional, rotatable sphere.
Some computer graphics systems attempt to provide a sense of a three-dimensional sphere, rather than a two-dimensional circle. The "arcball" of Shoemake, for example, provides additional highlightable arcs on the circle in order to emulate a three-dimensional sphere. Other systems give the impression of a sphere without depiction of a circle in the plane of the screen.
What is needed, therefore, is a computerized graphics system which enables the user better visually to anticipate and appreciate three-dimensional movement of a displayed object.